Menthone exists in the form of two stereoisomers, menthone (I) and isomenthone (II), each of which in turn exists in the form of two enantiomers.

Both stereoisomers occur in various essential oils, particularly in oils of the species Mentha. Menthones exhibit a typically minty note, whereas isomenthones have a slightly musty odor. Industrial menthones are often mixtures of the isomers in varying compositions. Menthone and isomenthone are used for synthetic peppermint oils and peppermint bases (source: K. Bauer, D. Garbe, H. Surburg, Common Fragrance and Flavour Compounds, 4th Edition, Wiley-VCH).
Various methods for preparing menthone are known in the literature:
(−)-Menthone may be extracted by distillation from so-called dementholized cornmint oil, a residue from the isolation of (−)-menthol from the essential oil of wild or corn mint (Mentha arvensis). Dementholized cornmint oil comprises 30-50% of (−)-menthone (source: K. Bauer, D. Garbe, H. Surburg, Common Fragrance and Flavour Compounds, 4th Edition, Wiley-VCH). Due to the natural origin, menthone is, however, also subject to cyclical fluctuations leading to price fluctuations, exactly as in the case of menthol. Moreover, due to LD-production, a reduction in the proportion of (−)-menthol from natural sources also decreases the availability of the dementholized cornmint oil.
U.S. Pat. No. 3,124,614 describes the synthesis of menthone by hydrogenation of thymol in the presence of Pd/C catalysts. However, only a mixture of all stereoisomers is accessible by this route, i.e. a mixture of rac-menthone and rac-isomenthone. (−)-Menthone may also be prepared by oxidation of (−)-menthol. All established methods for the oxidation of secondary alcohols are possible here, for example oxidizing agents based on toxic heavy metals (e.g. chromic acid or dichromate/sulfuric acid) or else oxygen or air in the presence of a catalyst (e.g. N-oxyl compounds, WO 2012008228).
WO 2005/085160 A1 describes the preparation of menthone by dehydrogenation of menthol in the presence of a catalyst in the gas phase. In this manner, a mixture of menthone and isomenthone is obtained. The dehydrogenation of isopulegol to isopulegone is likewise mentioned, but not the conversion of isopulegone to menthone.
A similar gas phase dehydrogenation of menthol to menthone is also described in DE 4236111 A1.
A common feature of all these publications is that the preparation of menthone starts from menthol. In terms of atom economy, however, it would be better to prepare menthone starting from isopulegol.
The direct synthesis of menthone from isopulegol with copper catalysis in the gas phase is described by W. Treibs and H. Schmidt in Berichte der Deutschen Chem. Gesellschaft 1927, 60B, 2335-41. Under the conditions described therein, a considerable amount of thymol forms (35%). The menthone is obtained as an unspecified mixture of (−)-menthone and (+)-isomenthone. Nickel catalysts on the other hand, which may also be used for example for the dehydrogenation of menthol to menthone, lead to the elimination of water from the isopulegol.

The problem to be addressed by the current invention is therefore to make available an improved method for preparing menthone.